1. Field of the Invention
The present invention relates to a diagnosing apparatus capable of detecting a trouble or defect associated with an automatic transmission system of a motor vehicle.
2. Discussion of the Related Art
Some motor vehicles use an automatic transmission system including (a) an automatic transmission incorporating a plurality of frictional coupling devices, (b) a plurality of solenoid-operated valves which are operated according to shift commands, and (c) an electronic control device generating the shift commands for controlling the operating states of the frictional coupling devices, to automatically place the automatic transmission in a selected one of the operating positions. The operating position to be selected is specified by the shift position commands. Usually, such automatic transmission system is equipped with a detecting circuit for detecting electrical disconnection and/or short circuiting of solenoid coils of the solenoid-operated valves. The detecting circuit generates signals indicating such defects that cause a failure of the automatic transmission to normally operate according to the shift commands received from the electronic control device. However, the absence of the electrical disconnection and short circuiting of the solenoid coils of the solenoid-operated valves does not necessarily mean normal functioning of the solenoid-operated valves. For instance, the detecting circuit is not capable of detecting other troubles or defects associated with the automatic transmission system, such as sticking of plungers or spools of the valves due to foreign matters contained in the working fluid or misalignment of the plungers. Such defects lead to failure of the spools to be normally moved for controlling the frictional coupling devices.
JP-A-1-172663 discloses a diagnosing apparatus adapted to detect the input and output shaft speeds of an automatic transmission, calculate an estimated input shaft speed of the transmission on the basis of the detected output shaft speed and the gear ratio of the currently selected operating position of the transmission, and compare the estimated input shaft speed with the detected actual input shaft speed, for determining whether the automatic transmission is normally functioning.
The known diagnosing apparatus indicated above uses speed detecting means for detecting the input shaft speed of the automatic transmission, which includes, for example, a gear which rotates with the input shaft and which have teeth to be detected, and a magnetic sensor adapted to detect passage of the teeth of the gear. To provide the automatic transmission with such gear and sensor, the transmission must be modified in design. This modification tends to increase the size and weight of the transmission, and also the cost of manufacture. On the other hand, the size, weight and cost of the transmission must be minimized to meet recent requirements in the manufacture of automobiles. Although the speed of the vehicle engine may be substituted for the input shaft speed of the transmission, the engine speed generally deviates from the input shaft speed of the transmission, due to a speed difference therebetween in the presence of a fluid-actuated power transmitting device such as a torque converter usually provided in the transmission. In other words, the use of the engine speed in place of the input shaft speed of the transmission does not assure accurate diagnosis of the automatic transmission.
The known detector or diagnosing apparatus indicated above is not capable of localizing or pin-pointing a defect or trouble detected, and requires considerable time and efforts to find out a source or cause of the defect.